Number entering illegal gang have tattoos to start with (5)
I believe the answer is:
eight
'number' is the definition.
(eight is a kind of whole number)
'entering illegal gang have tattoos to start with' is the wordplay.
'to start with' suggests taking the first letters.
The initial letters of 'entering illegal gang have tattoos' is 'EIGHT'.
(Other definitions for eight that I've seen before include "Several in a row" , "1,001 - 993" , "Oxford crew, perhaps?" , "2 cubed" , "Number etched by a skater?" .)